(1) Field of the Invention
The invention relates to the general field of photoprinters, more particularly to print heads based on field emission devices.
(2) Description of the Prior Art
FIG. 1 is a schematic illustration of a typical photoprinter of the prior art as described by, for example, Oka et al. (U.S. Pat. No. 4,794,062 December 1988). An electrostatic drum 1, having a cylindrical shape and seen end-on in the figure, is capable of rotation about an axis 2. With the drum rotating in clock-wise direction (in this example) the photoprinting process begins with a mechanical cleaning of the drum surface by suitable mechanism 3 such as, for example, a scraper blade.
The freshly cleaned surface is then exposed to electrostatic discharge unit 4 following which it receives a uniform electrostatic charge from charging unit 5. The charged surface now passes beneath light image source 6, said light being focused onto the drum by focusing means 7. Most commonly, light image source 6 is an array of light emitting diodes (LEDs).
Wherever light from source 6 strikes the drum's surface, the local electrostatic charge is neutralized so that a charged negative image of the pattern formed by the LEDs remains on the drum's surface. As the drum continues its rotation, it passes toner dispenser 8 where toner is electrostatically attracted to said charged image. Finally, toner is transferred, with little or no loss of image quality, to paper 9 which is being pulled past the drum by rollers 10.
A closeup view, in isometric projection, of a typical LED print head is shown in FIG. 2. The actual light source is linear array 26 of LEDs. These are driven by Integrated Circuits (ICs) such as 21. Excess heat is removed through heat sink 22. The entire array is protected by means of glass cover 27. Print heads of this type are relatively expensive and deliver light of relatively low intensity. This in turn slows down the rate at which image information can be transferred to the drum.
Cold cathode electron (or field) emission devices (FEDs) are based on the phenomenon of high field emission wherein electrons can be emitted into a vacuum from a room temperature source if the local electric field at the surface in question is high enough. The creation of such high local electric fields does not necessarily require the application of very high voltage, provided the emitting surface has a sufficiently small radius of curvature.
The advent of semiconductor integrated circuit technology made possible the development and mass production of arrays of cold cathode emitters of this type. In most cases, cold cathode field emission displays comprise an array of very small conical emitters, or microtips, each of which is connected to a source of negative voltage via a cathode conductor line or column. Another set of conductive lines (called gate lines) is located a short distance above the cathode lines and is orthogonally disposed relative to them, intersecting with them at the locations of the microtips, and connected to a source of positive voltage. Both the cathode and the gate line that relate to a particular microtip must be activated before there will be sufficient voltage to cause cold cathode emission.
The electrons that are emitted by the cold cathodes accelerate past openings in the gate lines and strike a conductive phosphor screen that is located a short distance from the gate lines. In general, a significant number of microtips serve together as a single pixel for the total display. Note that, even though the local electric field in the immediate vicinity of a microtip is in excess of 1 million volts/cm., the externally applied voltage is only of the order of 100 volts.
Field emission displays are normally intended for human viewing rather than as light sources in photoprinters. We are unaware of any prior art that discloses their use as print heads or similar application.